Bach chose to write this in C sharp major with a key signature of 7 sharps rather than the enharmonic equivalent of D flat major with 5 flats. The use of the adjective ‘well-tempered’ in the title implies a tuning in which all the pieces in the collection would sound well; this is not the same as equal temperament, in which each note is tuned slightly impurely so that there is an equal interval between each semitone. Both prelude and fugue are light hearted pieces which belie the complication of reading in this key!